Precise measurement of current is necessary in many applications. It is desirable to develop a high-precision, non-interceptive current (or flux) measuring scheme based on a digital SQUID (Superconducting QUantum Interference Device) technology. The advantage of using SQUIDs, in general, and digital SQUID, in particular, is that this technology not only improves the dynamic range of measurement systems, but also reduces the cost and complexity of supporting, peripheral electronics. Another desirable feature is that the digital nature of the outputs lends itself to digital signal processing, naturally.
The following examples illustrate some applications in a nuclear facility accelerator facility where high resolution over a wide range of signal amplitude is desirable.
1) Beam Current Monitor (BCM)
The average circulating beam current must be measured to a very high precision (10xe2x88x924-10xe2x88x926). This measurement can provide important information about particle losses in a beam. Even small deviations in the average beam current must be diagnosed to ensure stability of the beam. Depending on the particulars of an accelerator, the maximum average beam current can range from 100 xcexcA to 100 mA. Therefore, a current resolution of about 10 nA is needed.
2) Beam-in-Gap Monitor (BIG)
The measurement of gap current, which is 104-105 times smaller than the peak bunch current, is another demanding application. In the SNS (Spallation Neutron Source) ring, the bunch currents can be as high as 100 A. However, gap currents in the mA range must be measured accurately to determine the fraction of the beam outside the bunch. Since the gap duration is 250-300 ns, the measurement bandwidth must be at least several MHz. This is an extremely challenging task in terms of both dynamic range (105) and slew rate of the measurement instrument.
3) Beam Polarization Measurement
Direct measurement of beam polarization can be made if the magnetic flux produced by the aligned dipole moments in a polarized beam can be measured. This is difficult because the magnetic fields produced by the charged particle are 1010-1015 times larger. An extremely high resolution measurement instrument is needed to measure such a small signal in the presence of an overwhelming background. By careful design of the pick-up coil, the background signal can be reduced to about 108 times the polarization signal. Still, detecting this signal will require about 27 bits of resolution.
Applicant""s invention is directed to a subranging architecture using digital SQUID (Superconducting QUantum Interference Device) technology to design systems with larger dynamic range, higher resolution and larger bandwidth than existing systems. Systems embodying the invention may be used to manufacture (current or magnetic flux) measuring instruments useful in a diverse range of applications requiring high resolution and the sensing of signals over a wide range of signal amplitude.
An analog-to-digital converter (ADC) embodying the invention includes circuitry for supplying an analog input signal to an input coil having at least a first inductive section and a second inductive section. A first superconducting quantum interference device (SQUID) is coupled to the first inductive section and a second SQUID is coupled to the second inductive section. The first SQUID is designed to produce xe2x80x9ccoarsexe2x80x9d (large amplitude, low resolution) output signals and the second SQUID is designed to produce xe2x80x9cfinexe2x80x9d (low amplitude, high resolution) output signals in response to the analog input signals. The output signals of the first SQUID are coupled to a first comparator having an output for producing a first quantized output signal which is coupled back to the input coil. The output signals of the second SQUID are coupled to a second comparator having an output for producing a second quantized output signal which is also coupled back to the input coil.
In one embodiment of the invention first and second clock signals are respectively applied to the first and second comparators for enabling one of them at a time. In certain embodiments, the first and second inductive sections are connected in series. Typically, the inductance of the second inductive section is greater than the inductance of the first inductive section for causing the first SQUID and the first comparator to produce a xe2x80x9ccoarsexe2x80x9d output and the second SQUID and the second comparator to produce a xe2x80x9cfinexe2x80x9d output.
In an embodiment of the invention each one of the first and second comparators is a superconducting comparator; with each one of the first and second comparators being responsive to an output signal of its corresponding SQUID and to a clock signal for producing first and second comparator complementary output signals. The first output signal of each comparator is coupled to a first write gate and the second output signal of each comparator is coupled to a second write gate. The first write gate produces a comparator output signal of one binary value and the second write gate produces a comparator output signal of a second binary value, with the output signals of the first and second comparators being fed back to the input coil and to processing circuitry.
In a subranging superconducting ADC embodying the invention each one of the first and second comparators includes a digital output, with the digital output of the first comparator defining the more significant bits and the digital output of the second comparator defining the lesser significant bits of the digitally converted signal.
The digital output of each comparator may be coupled to an up-down counter, with each up-down counter having an output coupled to an accumulator whose output is coupled to a processor for processing the data digitally. The processor may include a digital filter and data acquisition and software for analyzing the data.
A subranging superconducting ADC embodying the invention may also include a first circuit arrangement for producing xe2x80x9ccoarsexe2x80x9d (high amplitude, low resolution) output signals and a second circuit arrangement for producing xe2x80x9cfinexe2x80x9d (low amplitude, high resolution) output signals. The first circuit arrangement includes circuitry for supplying an analog input signal to a first inductor for inductively coupling the analog input signal to a first SQUID based circuit. An output of the first SQUID based circuit is coupled to a first digital filter for producing a xe2x80x9ccoarsexe2x80x9d digital signal. The second circuit arrangement includes circuitry for subtracting an output of the first SQUID based circuit from the analog input signal and applying the resultant analog signal to a second SQUID based circuit. An output of the second SQUID based circuit is supplied to a second digital filter for producing a xe2x80x9cfinexe2x80x9d digital signal.
In one embodiment, the circuitry for subtracting an output of the first SQUID based circuit from the analog input signal includes an analog delay line for coupling the analog input signal to a second inductor and a digital variable delay network having an input connected to the output of the first SQUID based circuit and having an output connected to the input of a digital-to-analog converter (DAC) with the output of the DAC being connected to a third inductor and supplying a signal thereto tending to cause the output of the first SQUID based circuit to be subtracted from the analog input signal developed across the second inductor.